VB.NET container that doesn't add different items

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VB

containers

There is a container in VB.NET that when you add an item that already exists then nothing happens (it is not added )
for example if the container variable is named a then

a.add(1) a.add(1) a.add(2)

will result in "a" having two items (1,2 ) and not three(1,1,2)
the problem is that I forgot the object's name
any help ?
thank you !